The present invention pertains to a motor vehicle tire with a tread profile for a superior grip on snow and ice.
For this purpose, the tread profile customarily contains incisions that essentially extend parallel to one another within profile positives. The expression xe2x80x9cprofile positivesxe2x80x9d is used here as a general term for xe2x80x9cpadsxe2x80x9d or xe2x80x9cblocks,xe2x80x9d i.e., for elevated profile regions that are not only limited in the axial direction, but also in the circumferential direction, as well as for xe2x80x9cribs,xe2x80x9d i.e., for elevated profile regions that are only limited in the axial direction and extend over the entire circumference.
Incisions of this type are usually, and in the present invention, distributed similar to screen lines. This means that the distance between the respective incisions is essentially constant. In this context, the term xe2x80x9cessentiallyxe2x80x9d means that the greatest distance between two incisions is not larger than the smallest distance between two incisions by more than 20%, with this definition applying to all pairs of two respectively adjacent incisions within the same profile positive. In this case, the average distance between two adjacent incisions should lie between 3 mm and 7 mm. It is preferred to maintain the incision spacing in each profile positive exactly constant within the above-described range of dimensions, at least for tires that are not intended for only one rotating direction.
It is known to design a series of incisions in such a way that they form, when viewed from the top, at least one almost perpendicularly staggered stair-like section, namely from the Semperit winter tire xe2x80x9cDirection-Gripxe2x80x9d that is, for example, described on pages 22 and 23 of xe2x80x9cGummibereifungxe2x80x9d 11/1988. In this context, the term xe2x80x9calmost perpendicularlyxe2x80x9d refers to an angle between 75xc2x0 and 105xc2x0. The above-mentioned article also states that the length of the staggered section is no longer than half the length of the main section of the incision.
Tire experts are familiar with the fact that the stiffness of a positive without incisions referred to a radially applied force is greater in the center of the positive than on the edge of the positive. Consequently, the surface pressure is greater in the center of the positive than on the edge of the positive.
The experts are also familiar with the fact that the coefficient of friction xcexc increases as the pressure decreases. If the contact surface cannot be enlarged and the mass to be supported cannot be reduced, the pressure should be as constant as possible at least over the available contact surface. In connection with the facts stated in the preceding paragraph, this has led tire experts to the conclusion that measures should be taken for realizing positives similarly soft in the center and on the edge. Although DE-AS 24 32 363 (1981) aims to achieve an improved water discharge from the ground contact surface, a certain softening of the profile pad centers is already achieved with the measures described in this publication.
Information on a more effective measure, namely a pocket incision in the pad center which does not extend through the edge of the pad, can be obtained from German Utility Model M 93 01 038 (1993) or the article on Avon Tire Type CR38 on page 114 of xe2x80x9cGummibereifungxe2x80x9d 7/1996.
Another measure for concentrating a pad softening caused by incisions on the pad center, in particular, on the second row of pads from the left, can be obtained from the article on the tire type xe2x80x9cEnergyxe2x80x9d by Michelin on page 94 of xe2x80x9cGummibereifungxe2x80x9d 1/1997. This measure consists of semi-pocket incisions that approximately extend in the axial direction in connection with one respective intersecting longitudinal incision. The softening of the pad is at its greatest at the incision intersections.
However, all aforementioned measures have the disadvantage that the flexural strength in the pad center is also reduced along with achieving the desired softening of the pad center referred to radially applied forces, i.e., along with the desired reduction in the compressive rigidity in the pad center. This means that tangential forces, e.g., propulsive or brake forces, result in an increased tilting of the pad sections that are separated from one another by the incisions. Although this slightly improves the grip on snow-covered roads, the coefficient of friction on icy roads which is particularly critical in situations of this type significantly deteriorates because the aforementioned tilting concentrates almost the entire surface pressure on the very small edge surfaces. In other words, the reduced flexural strength eliminates the advantage of a reduced compressive rigidity in the pad center with respect to the most critical aspect of a winter tire, namely the grip on icy roads.
Therefore, an objection of the present invention is to develop a profile geometry that reduces the compressive rigidity in the pad center and results in a higher degree of flexural strength.
The above and other objections of the invention can be achieved with a motor vehicle tire with a tread profile for a superior grip on snow and ice
wherein incisions (E1, E2, E3) that essentially extend parallel to one another are arranged within profile positives (=pads and/or ribs),
wherein the distance (a) between adjacent incisions (E) of the same profile positive preferably is constant or deviates by no more than 20%,
wherein the average distance (am) between two adjacent incisions (E) lies between 3 mm and 7 mm,
wherein a series of incisions (E)xe2x80x94preferably all incisions (E)xe2x80x94contain at least one almost perpendicularly staggered stair-like section (E) when viewed from the top, and
wherein the length (lEH) of the staggered section (Ev) amounts to no more than half the length (lEH) of the main section (EH) of the incision (E), and wherein
the staggered sections (EV) are offset relative to one another in adjacent incisions (E1, E2) of the same profile positive,
namely such that a rubber web (G) is respectively situated between said staggered sections (EV), wherein the length of the rubber web transverse to the length of the two adjacent main sections (EH1, EH2) only lies between 30% and 70% of the distance (a) between the main sections (EH1, EH2).
It is important to note the tire according to the invention is characterized by the fact
that the staggered sections are offset relative to one another in adjacent incisions of the same profile positive,
namely such that a rubber web is respectively situated between said staggered sections, wherein the length of the rubber web transverse to the length of the two adjacent main sections only lies between 30% and 70% of the distance between the main sections.
The length of said web along the two adjacent main sections preferably should lie between 0.6 times and 2 times the distance between the main sections.
The web has the desired lower compressive rigidity due to the vicinity of incisions on all sides; its flexural strength is, however, barely reduced due to the angular connection to the adjacent positive sections between the main sections of two respectively successive incisions.